Support external events for interact with conversation resource

[caly-amigo]

Summary

• fix conversation.interact_with_conversation request encoding so multipart bodies are valid in real HTTPX sync/async clients
• support external event context arrays in multipart form fields:
  • external_event_message_content
  • external_event_message_timestamp
• keep repeated field semantics required by OpenAPI for external event context
• update integration tests so external event payloads include one timestamp per external event and timestamps are after the latest interaction/message time in the conversation

Root Cause

The prior implementation changed request_kwargs["data"] to a tuple-list to preserve repeated fields. That shape worked in mocked unit tests but produced invalid multipart streaming behavior in live HTTPX requests.

Implementation Details

• switched to building multipart fields via files entries, including non-file fields as (None, value)
• appended the actual recorded_message file part to the same multipart files payload
• updated tests to assert multipart fields from files

Validation

• Local:
  • uv run ruff check .
  • uv run ruff format --check .
  • uv run pytest tests/resources/test_conversation.py -q
• CI (latest commit d817b30):
  • Test: ✅ https://github.com/amigo-ai/amigo-python-sdk/actions/runs/22072950150
  • CodeQL: ✅ https://github.com/amigo-ai/amigo-python-sdk/actions/runs/22072948928